The field of this invention relates to chemical operations and processes carried out in semifluidized beds. More particularly, this application relates to mass transfer operations, such as adsorption and ion exchange, using porous granules in semifluidized beds.
Mass transfer operations in which a removable component is transferred from a liquid or a gas into porous granules can be carried out with the granules arranged in fixed beds, fluidized beds, or semifluidized beds. In the latter bed arrangement, a fixed bed portion is maintained above a fluidized bed portion in an upflow fluidized bed-type apparatus. By a vertical adjustment of the position of the sieve plate at the top of the bed, the size of the fixed bed portion can be varied with a corresponding increase or decrease in the fluidized bed portion. Because of this feature, semifluidized beds have also been referred to as "convertible" beds, the fluidized and fixed bed portions being selectively convertible.
Semifluidized bed contacting was originated and patented by Liang-tseng Fan and Chen-yung Wen (See U.S. Pat. No. 3,374,052, and A.I.Ch.E. Journal (1959), Vol. 3, No. 3, pp. 407-409.) Various uses for semi-fluidized bed contacting, have been proposed, viz. ion exchange processes, drying of gases, adsorption of contaminants from liquids, etc. The mathematical characteristics of semi-fluidized beds have been delineated by Fan and Wen: A.I.Ch.E. Journal (1960), Vol. 6, No. 3, 482-487; and A.I.Ch.E. Journal (1963), Vol. 9, No. 3, pp. 316-320.
In recent years, a number of special uses of semifluidized beds have been patented, including filtration (U.S. Pat. No. 4,157,959) and wastewater treatment in biological reactors (U.S. Pat. No. 4,322,296). However, although the potential desirability of using semifluidized beds for mass transfer operations, such as adsorption or ion exchange, has been recognized, such applications have received very little consideration since Fan and Wen originally disclosed their concept for semifluidized bed contacting.
Adsorption is a mass transfer unit operation used in chemical processing, water and wastewater treatment, air pollution control, etc., to remove materials present in small concentrations in fluids. This operation has generally been carried out in fixed beds by contacting large volumes of fluid with small quantities of adsorbent. Since large volumes of fluid are usually being processed, the amount of pump energy required to move the fluids (gas or liquids) through the beds is a very significant economic factor. Potentially large energy savings might be made by carrying out such mass transfer operations in semifluidized beds. The pressure drop across a semifluidized bed containing the same amount of adsorbent as a fixed bed is much lower than that across a fixed bed. Therefore, lower pump pressures and energy consumption are required for treating the same volumes of fluid.
It has also been recognized that in applications involving interphase mass transfer in gas-solid or liquid-solid systems, operating efficiency can be improved by increasing the external mass transfer rate, viz. from the fluid phase to the solid phase. In a semifluidized bed, the porous granules in the fluidized portion of the bed are vigorously agitated due to the fluidization. The mass transfer rate should therefore be much higher under fluidizing conditions than in fixed bed transfer. Furthermore, the fixed bed portion (which may be of relatively small size) can serve as a finishing or polishing section. Consequently, both a high rate of treatment and thorough removal should be obtainable. Prior to the present invention, however, it was assumed that the total adsorption capacity of the semifluidized bed would necessarily be substantially lower and for the same volume of adsorbent as in a fixed bed. This has tended to limit consideration of semifluidized beds for mass transfer. Adsorption capacity is a very important factor when large volumes of fluid are being processed as is usually the case in commercial mass transfer operations.